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12.1.3 Seasonal variations

One of the noted differences between the seasonal VMR distributions is the airmass descent at the high latitudes in the spring, which is apparent when following the 3 and 4 ppb isolines from 60 N ownwards. These seasonal signature are also imprinted with the higher values in the MPV (not shown here). Thus, the high latitude lower stratospheric ozone abundance is influenced by the seasonal cycle of stratospheric transport (Ko et al., 1989; Rosenlof, 1999).

In fall the high VMRs ( 9 ppm) are extended up to 40 N in ASUR, whereas in the spring the same values are noted only up to 30 N. The simulations also show high values in the tropics in the fall as compared to the spring mixing ratios. However, the values in the ozone source region in the tropical stratosphere is more reasonably reproduced by SLIMCAT than

CTMB.It should be noted that the upper stratospheric simulations by SLIMCAT underpredict the measurements and theCTMB calculations.

Figure 12.3: The O3 vertical profiles from ASUR measurements compared to SLIMCAT and CTMB

ozone simulations. The numbers on the bottom of the figures represent the number of averaged profiles in each latitude section and the color codes indicate the respective seasons as shown by the legend.

Figure 12.3 illustrates the ozone profile comparison at different latitude sections for the two seasons. The latitude sections are defined as 5 S to 30 N for the tropics, 30 N to 60 N for mid-latitudes and 60 N to 90 N for the Arctic or high latitude . The same definition is used elsewhere in the chapter for referring the climatic regimes. In the tropics the seasonal variation is not pronounced in theASUR ozone, whereas the models show slightly higher values in the fall. Although the mixing ratios atAMV in the mid-latitude region show higher values in the fall, the lower stratosphericVMRs are higher in the winter/spring for both the measurements and the model simulations. In the Arctic, VMRs in the winter/spring season is higher except in the upper stratosphere, where the reversal found around 38 km. The middle stratospheric ozone low bias in the CTMB and the upper stratospheric ozone ’deficit’ in the SLIMCAT are also revealed. Though there are slight differences (inVMR) among the measurements and the simulations, the calculations capture the details of the latitudinal and seasonal variations very well.

Figure 12.4: Same as Figure 12.1, but for N2O.

12.2 N

O

12.2.1 Latitudinal variations

Figure 12.4 illustrates theASUR N₂O observations during the fall deployment. High mixing ratios are noted at the tropical low altitudes indicating their tropospheric origin (young air) and the tropical upwelling (Hall and Plumb, 1994). The high tropical mixing ratios give rise to steep horizontal gradients (Mahlman et al., 1986; Holton and Choi, 1988). The mixing ra-tios decrease with increasing altitude and latitude. A sharp gradient seen around 30 N likely to be caused by the subtropical barrier (Murphy et al., 1993; Minschwaner et al., 1996; Mote et al., 1996; Volk et al., 1996; Avallone and Prather, 1997; Plumb and Eluszkiewicz, 1999).

Another sharp gradient present at the mid latitudes around 45 N, is separating the air from mid and high latitudes (Rosenfield and Schoeberl, 1986). However, the gradients cannot be regarded as strength of the mixing barriers (Nakamura and Ma, 1997). The tracer isopleth (sameVMR level) is elevated at the low latitudes and crest fallen at the high latitudes because of the nature and influence of the meridional circulation (Plumb and Ko, 1992; Plumb, 2002).

Along the quasi-horizontal surfaces at the mid latitudes the isopleth is flat due to wave mixing (Randel et al., 1993). N₂O has an exponential decay with altitude in the stratosphere due to photolysis (Holton, 1986). TheSLIMCATcalculations for the September deployment is shown in the Figure 12.4(b). The general features observed by ASUR are also reproduced by the model quite well. The calculated values are slightly lower in the tropical lower stratosphere.

TheCTMB calculations for the same time period is depicted in Figure 12.4(c). As seen from the observations and from theSLIMCAT simulations, the model reproduces the measured fea-tures reasonably. However, the calculations show slightly lower values in the tropical lower stratosphere and higher values in the mid and high latitude upper stratosphere.

Figure 12.4(d) shows the observed N₂OVMR in the winter/spring season. The main latitu-dinal behavior is the same as discussed previously. Both the subtropical and the high latitude gradients are steeper in the spring. The SLIMCAT run for the same time period is given in Figure 12.4(e). The gradients are well reproduced and the mixing ratios are comparable. Fig-ure 12.4(f) shows the CTMB results for the same time period. The observations show high mixing ratios at a latitude from 5 S to 20 N, whereas in the model, the area is reduced from 5 S to 3 N, which is a noted difference between the measurements and the calculations. The

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Figure 12.5: Same as Figure 12.2, but for N2O.

simulated gradients separating the low, mid and high latitudes (subtropical and polar vortex mixing barriers respectively) are not as steep as observed, which is another prominant dis-crepancy. The seasonal variations will be discussed in Section 12.2.3.

12.2.2 Longitudinal variations

Figure 12.5 compares the measured longitudinal variations with theSLIMCAT and the CTMB

calculations. As discussed in Section 12.1.2, the variations from 60 W to 0 E are due to the high latitude longitudinal survey around 65 N. The latitudinal variations are shown by the ripples in contours from 0 E to 20 E. The variations in the tropics (20 E to 60 E) are not pronounced because of the effect of zonal winds. The SLIMCAT simulations reproduce the measured longitudinal variations very well. The given input initial climatology has no longi-tudinal variation in CTMB. As the model needs about 3 years to get adjusted for simulating the N₂O distribution, the long run also reduces the influence of the initial profiles on the final results of the model. However, the model is successful in imitating the observed longitudinal variabilities.

Figure 12.6: Same as Figure 12.3, but for N2O.

12.2.3 Seasonal variations

Since N2O is a tracer, a more profound seasonal difference in mixing ratio is expected. As planetary wave activity is higher in winter, the subtropical gradient is also steeper in winter (Hirota and Shiotani, 1983; Randel, 1988; Dunkerton, 1989). The observed gradients clearly show the seasonal cycle of the steepness in the subtropical gradient. The observations also show the airmass subsidence at the high latitudes during the winter/spring season with rela-tively small mixing ratios (Figure 12.4). Since the stratospheric transport is higher in winter, an increased tropical upwelling and an increased high latitude descent are apparent in both the measurements and in the calculations (Rosenlof and Holton, 1993; Holton, 1995; Rosenlof, 1995; Appenzeller et al., 1996). The diabatic descent at the high latitudes is estimated to be roughly about 4-6 km as following the N₂O isopleths. The seasonal variations (the difference in the tropical upwelling and the high latitude descent) are clearly seen in the longitudinal dis-tributions of the molecule as well (Figure 12.5). TheSLIMCATand theCTMBcalculations rea-sonably reproduce the observed seasonal features. However, theSLIMCATcalculation matches better with the observations as the gradients andVMRs are closer to the measurements.

Figure 12.6 illustrates the nitrous oxide profile comparisons at different climatic regimes.

In the spring the ASUR mixing ratios at the tropical latitudes are slightly higher in the lower stratosphere, indicates the high tropical airmass uplift in winter/spring season. However, in the upper stratosphere the fall values seem to have higher values. At the mid and high latitudes the fall mixing ratios are higher than those of the winter/spring due to increased stratospheric transport. The modeled profiles are consistent with the latitudinal and seasonal pattern of the measured profiles.

Figure 12.7: Same as Figure 12.1, but for HCl.

12.3 HCl

12.3.1 Latitudinal variations

Figure 12.7(a) depicts the observed HCl mixing ratios in the fall. Since HCl is a quasi-inert tracer in the stratosphere, its distribution partially depends on dynamics of the region (Rasch et al., 1995). A rapid increase in Cl_y mixing ratios with height is found in the lower strato-sphere where photolysis of chlorine source gases is the most efficient. So a similar pattern is expected for HCl mixing ratios with the stratospheric altitudes as it is one of the reservoir gases of chlorine in the stratosphere. Since the photolysis is very efficient in the tropics, the

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